Dynamic calibration of pass voltage (Vpass) and wordline read-verify voltage (Vwlrv) is performed during certain memory operations, such as page read, to account for charge loss in distributions with ageing, for example. Existing systems that dynamically calibrate such voltages often perform calibration as a function of the average string current in the memory sub-blocks. Drawbacks of approaches like this can include undesired sensitivity to temperature changes and/or time of programming requirements. For example, since temperature compensation is only done at the desired sense current or sense Vt (threshold voltage), i.e., the voltage at which the cells are sensed as erased or programmed, temperature-induced variation of cell current from cells not at sense Vt can cause inaccuracy in voltage calculation and resulting calibration. Moreover, if different pages or sub-blocks are programmed at different times, inaccurate calibration of dynamic pass voltage or wordline read-verify voltage may occur due to high currents from unselected sub-blocks. This is because unselected sub-block segmentation or turn-off cannot be done until the final pass voltage is determined, which happens after the comparator flips indicating completion of calibration.
Approaches that dynamically calibrate pass voltage as a function of current passing through sub-blocks may also suffer from undesired fluctuation based on the pattern of programming. Here, for example, variation in quantity and/or location of pages programmed may affect calibration, since the average string current expected is a function of the number of pages programmed per block.
The disclosed technology provides improved calibration of pass voltage or wordline read-verify voltage by reducing sensitivity to temperature, pattern, and time of programming issues and/or addressing other deficiencies in the prior art.